The present invention refers in general to antennas and in particular to indoor antennas.
Efficient electromagnetic wave propagation, within al indoor environment, requires special attention to antenna pattern aid polarization. The effect of these two factors may be intuitively understood. First, due to the “Near-Far” effect, the antenna needs to emphasize power density towards relatively farther away (distant) users while de-emphasizing power density directed towards relatively close users. Second, in an indoor environment, wave polarization is impacted by reflections, diffraction and scattering, thus creating a significant horizontal component.
Wide band antenna operation may be achieved by many methods and antenna structures. Most, such as Yagi, log periodic or fractal element-based antennas, require relatively complicated structures which are expensive to implement. Elliptical and circular polarization can also achieved by the use of three dimensional radiators such as conical spiral elements, as described for example in U.S. Pat. No. 4,675,690. However, such elements are expensive to produce.
A family of monopole antennas (sometimes called “inverted F antennas”), to which elements in the present invention bear some distant resemblence, is known, see e.g. [1] Y. Hwang, Y. P. Zhang, and T. K. C. Lo “Planar inverted F antenna loaded with high permittivity material”, IEEE Electronic Letters, vol. 31, no. 20, September 1995; [2] C. R. Rowell and R. D. Murch, “A Capacitively Loaded PIFA for Compact Mobile Telephone Handsets”, IEEE Trans. Antenna and Prop. Vol. 45, no. 5, May 1997; [3] K. L. Wong and K. P. Yang, “Modified planar inverted F antenna”, IEEE Electronic Letters, vol. 34, no. 1, January 1998; [4] C. M. Su, K. L. Wong, W. S. Chen, and Y. T. Cheng, “A Microstrip Coupled Printed Inverted-F Monopole Antenna”. Microwave and Optical Techn. Letters, vol. 43 no. 6 December 2004; and [5] H. Elsadek, D. Naslhaat and H. Ghall, “Multiband Miniaturized PIFA for Compact Wireless-Communication Applications”, Microwave and Optical Technol. Letters, vol. 42, no.3, August 2004, all of which are incorporated herein by reference. These antennas are usually characterized by narrow band operation due to the strong coupling between the physical length of the antenna and its operating wavelength. However, the demanding wireless market requires continued miniaturization and increased operating bandwidth. The literature reports several solution techniques for miniaturization as well as multiband operation. Nevertheless these solutions, which use several resonance frequencies established by parasitic and multi-element construction, are not truly wide band. These solutions also lack stable radiation patterns over their resonance frequency (see FIG. 3 in (ref. [1], FIG. 7 in ref. [2] and FIG. 9 in ref. [5]), thereby enforcing a non-optimal frequency and spatial coverage.
Some attempts have been made to enlarge the frequency bandwidth, see e.g. [6] N. P. Agrawall, G. Kumar, K. P. Ray, “Wide Band Planar Monopole Antennas”, IEEE Trans. Antenna and Prop. Vol. 46, no. 2, February 1998; [71] J. Liang, C. C. Chiau, X. Chen, C. G. Parini, “CPW-fed circular ring monopole antenna”, IEEE Antenna and Prop Int. Symp. 2005; [8] and G. Chi, B. Li, D, Qi, “A Dual-frequency Antenna Fed by CPW”, IEEE Antenna and Prop Int. Symp. 2005, all of which are incorporated herein by reference. The antennas described in [6] and [7] are broadband, however their azimuthal pattern variation exceeds 7dB, and therefore they cannot be considered as omni-directional. The antenna in [8] lacks both wide bandwidth (more then 50%) and omni-directional radiation pattern.
In view of the disadvantages of known antennas in terms of bandwidth and omni-directional operation, there is a need for, and it would be beneficial to have an antenna that does not suffer from these disadvantages. In particular, it would be advantageous to have antennas with circular polarization and/or a significant horizontal component for indoor use.